1. Field of the Invention
The present invention relates to an optical scanner using an image forming reflecting mirror, etc.
2. Description of the Related Art
In an optical scanner, a light beam is deflected at an equal angular velocity and is converged as a light spot on a scanned face to perform an optical scanning operation on the scanned face. Various kinds of optical scanners each having such a structure are generally known in association with an optical printer, etc. An f.theta. lens generally constitutes an optical system for converging the deflected tight beam as a light spot on the scanned face and performing the optical scanning operation using the light spot at an equal speed. However, an image forming reflecting mirror having a linearity correcting function has been recently proposed instead of the f.theta. lens. For example, Japanese Patent Application Laying Open (KOKAI) No. 1-200221 shows such an image forming reflecting mirror.
In the optical scanner using such an image forming reflecting mirror, a scanning line as a moving locus of the light spot to be linear tends to be curved. Curving of the scanning line is called as a curve in the scanning line.
In the optical scanner using the above image forming reflecting mirror, the deflected light beam incident to the image forming reflecting mirror and the deflected light beam reflected on the image forming reflecting mirror are located on the same side with respect to the image forming reflecting mirror. Accordingly, it is necessary to use an optical arrangement in which these reflected light beams are separated from each other and the reflected deflected light beam is not returned to the side of a light source, but is guided to a side of the scanned face. In such an optical arrangement, there is a case in which the scanning line is necessarily curved.
Such a curve in the scanning line caused by the optical arrangement can be corrected by shifting a position of the image forming reflecting mirror in a cross-scan corresponding direction or inclining the image forming reflecting mirror to such an extent that no practical problems are caused. However, a very high accuracy in arrangement of each of optical elements arranged on an optical path from the light source to the scanned face is required to correct this curve as designed. Therefore, when errors in accuracy in arrangement of the optical elements are accumulated, the scanning line is greatly curved.
For example, such a curve in the scanning line causes a great problem in a two-color printer, etc. for performing a writing operation using a separate optical scanning operation every color information. For example, when a scanning line L is set to an ideal scanning line in FIG. 6, there is a case in which scanning lines L.sub.1 and L.sub.2 for writing respective chromatic information of two colors are curved in opposite directions as shown in FIG. 6. In such a case, a shift of (a.sub.1 +a.sub.2) is caused at its maximum between the written chromatic information in both end portions of an optical scanning region. Accordingly, a so-called color shift is markedly caused.